1. Field of the Invention
The present invention relates to an organic electroluminescent device or element (hereinafter, also referred to as an “organic EL device”) which can be advantageously, for example, utilized as a planar light source or in display devices.
2. Description of the Related Art
Attention has been made to an organic electroluminescent device having a luminescent layer, i.e., light-emitting layer, formed from the specific organic compound, because it ensures a large area display device with low-voltage driving. To obtain an EL device with a high efficiency, Tang et al., as is reported in Appl. Phys. Lett., 51, 913 (1987), have succeeded in providing an EL device having a structure in which organic compound layers having different carrier transporting properties are laminated to thereby introduce holes and electrons with a good balance from an anode and a cathode, respectively. In addition, since the thickness of the organic compound layers is less than or equal to 2,000 Å, the EL device can exhibit a high luminance and efficiency sufficient in practical use, that is, a luminance of about 1,000 cd/M2 and an external quantum efficiency of about 1% at an applied voltage of not more than about 10 volts.
In the above-described high efficiency EL device, Tang et al. used a magnesium (Mg) having a low work function in combination with the organic compound which is basically considered to be an electrically insulating material, in order to reduce an energy barrier which can cause a problem during injection of electrons from a metal-made electrode. However, since the magnesium is liable to be oxidized and is instable, and also exhibits only a poor adhesion to a surface of the organic layers, magnesium was used after alloying, i.e., by the co-deposition of the same with silver (Ag) which is relatively stable and exhibits good adhesion to a surface of the organic layers.
On the other hand, the researchers of Toppan Printing Co. (cf, 51st periodical meeting, Society of Applied Physics, Preprint 28a-PB-4, p.1040) and those of Pioneer Co. (cf, 54th periodical meeting, Society of Applied Physics, Preprint 29p-ZC-15, p.1127) have had developments in the usage of lithium (Li), which has an even lower work function than that of Mg, and alloying the same with an aluminum (Al) to obtain a stabilized cathode, thereby embodying a lower driving voltage and a higher emitting luminance in comparison with those of the EL device using the Mg alloy. In addition, as is reported in IEEE Trans. Electron Devices., 40, 1342 (1993), the inventors of the present application have found that a two-layered cathode produced by depositing lithium (Li) alone with a very small thickness of about 10 Å on an organic compound layer, followed by laminating a silver (Ag) to the thus deposited Li layer is effective to attain a low driving voltage in the EL devices.
Recently, Pei et al. of Uniax Co. have proceeded to reduce a driving voltage of the EL device by doping a polymeric luminescent layer with a Li salt (cf. Science, 269, 1086 (1995)). This doping method is intended to dissociate the Li salt dispersed in the polymeric luminescent layer to distribute Li ions and counter ions near the cathode and near the anode, respectively, thus ensuring an in-situ doping of the polymer molecules positioned near the electrodes. According to this method, since the polymers near the cathode are reduced with Li as a donor dopant, i.e., electron-donating dopant, and thus the reduced polymers are contained in the state of radical anions, a barrier of the electron injection from the cathode can be remarkably reduced, contrary to the similar method including no Li doping.
More recently, the inventors of the present application have found that a driving voltage of the EL device can be reduced by doping an alkali metal such as lithium and the like, an alkali earth metal such as strontium and the like or a rare earth metal such as samarium and the like to an organic layer adjacent to the cathode electrode (cf. SID 97, Digest, P.775). It was believed that such reduction of the driving voltage could be obtained because a barrier in the electron injection from the cathode electrode can be notably reduced due to a radical anion state in the organic layer adjacent to the electrode produced by metal doping therein.
However, due to oxidation of the electrodes and other reasons, deterioration of the device can be resulted in the above-described EL devices using an alloy of Mg or Li as the electrode material. In addition, use of such alloy-made electrodes suffers from the limited selection of the material suitable for the electrodes, because the electrode material to be used has to simultaneously satisfy the requirement for the function as a wiring material. Further, the above-described two-layered cathode developed by the present inventors is unable to act as a cathode when a thickness of the Li layer is not less than about 20 Å (cf. IEEE Trans. Electron Devices., 40, 1342 (1993), and also suffers from a low reproducibility problem in the device production, because there is a difficulty in the control of the layer thickness, when the Li layer is deposited at a remarkably reduced thickness in the order of about 10 Å. Furthermore, in the in-situ doping method developed by Pei et al. in which the Li salt is added to the luminescent layer to cause their dissociation in the electric field, there is a problem with the transfer time of the dissociated ions to the close vicinity of the electrodes having a controlled velocity, thereby causing a remarkable retardation of the response speed of the devices.
Moreover, for the method including doping the metal as a dopant in the organic layer, there is a necessity to exactly control a concentration of the dopant during formation of the organic layer, because the doping concentration can affect on the properties of the resulting devices.